Shop by mAh/price and take time to read the reviews. Some cheaper batteries can have issues with fit and watt-hours capacity. A frequent complaint is not holding a charge or short useful life. I bought a replacement for a MBP17 at ChromeBattery in 12/2010 that is holding up well, I believe they also sell at Amazon now.

To check the condition of a Mac's battery: - Apple menu - About this mac - More Info ... - System Report ... (only needed on Lion on more recent) - Under Hardware > Power you'll find the Health Information e.g.:

Health Information: Cycle Count: 29 Condition: Normal

That's on my MBP - as you see not that old. Worn out batteries I've seen on the last generation of MBPs that had user replaceable batteries (2009 model) I'd expect to see well over 500 to 600 cycles before they go bad.

More modern MacBook Pro's do NOT have user replaceable batteries. (you can see it easily enough: the underside will have one or more latches if it's user replaceable, otherwise it's only screws).

The two most recent of those batteries are still being sold by apple - any apple store or authorized apple dealer should have them in stock. It's a trivial swap and you have an as new battery. The one where about 1/3 of the bottom is a cover: [store.apple.com...] The one where the battery sits in the middle of the machine: [store.apple.com...]

I'd not use just anything as these high capacity batteries can be dangerous. Most people do not realize it but modern batteries include firmware that talks to the motherboard/power circuits in order to keep it in good working order as well as SAFE. It's not like a battery is a high explosive, but these battery cells could "explode", leak nasty stuff, and/or start a fire if they get used in a wrong manner (that's what the firmware avoids). I'd NOT use any battery that's not designed by Apple for the MBP. Anything else before the battery - just too dangerous in my book.

That brings us to the economic wisdom of replacing a battery in a machine that's by now about 4 years old (or more). Sure you want to invest in it ? New macs are considerably faster/lighter/have better screens/more storage/...